I can’t speak for all travel agents but can certainly speak for myself. I’ve been on close to 30 cruises and most certainly know the difference between ships, ports, cabins, etc.
In response to the original question about using a TA, it depends on what type of traveler you are. TAs are very helpful putting together multiple types of travel experiences - most people don’t want to spend HOURS/DAYS researching and building their trips and then worrying if they put all the pieces together correctly, got the correct visas, have all their transfers, etc. They’re also an excellent resource if something should go wrong, as was previously mentioned in this thread.
If, however, you’re looking for a dirt cheap trip and hoping someone will give you a bunch of OBC/discounts, a TA will most likely only do that if you’re a close friend or they’re hoping for future, more lucrative, business from you.
As for the big box stores extreme rebating, the travel suppliers should be ashamed of themselves for doing business with them. Rebating is simply discounting and that’s against most suppliers policies. While I understand the advantage from a monetary standpoint, I wouldn’t want to have to rely on some online agent at the big box store if I got hurt in a foreign country, missed a flight, or had a travel supplier go bankrupt mid-trip and leave me stranded. That’s not worth a $300 in-club gift card to me. Plus, travel agents are small businesses (even the ones that work for large agencies are typically independent contractors) and I like to support small business every chance I get.